Image quality of printing in a printed material is deteriorated depending on change in environment such as temperature, humidity, etc., a state of consumable goods of a printing apparatus, the number of print sheets and the like at the time of printing. Therefore, a printing apparatus executes calibration (also referred to as process control) when specific conditions are satisfied, for example, with every XX days in a case where change in temperature or humidity is X or more, with every XXX sheets of printing and the like, and generates concentration correction information. Then, the printing apparatus generates print image data (bitmap) using the generated concentration correction information and prints the print image data, thereby preventing degradation of image quality of printing.
In calibration, processing described below is mainly executed. First, when the specific conditions (hereinafter, described as calibration execution conditions) are satisfied, a printing apparatus forms toner images for testing (referred to as toner patches) whose concentrations are different from each other on a surface of a photoreceptor drum or on an intermediate transfer member, detects reflection light from the toner patches using a concentration sensor such as an optical sensor, and measures concentrations of the toner images.
The printing apparatus then generates “concentration correction information” based on the measured concentrations. The concentration correction information is information used, for example, when a measured concentration of a toner image is low, in changing a processing content of RIP (Raster Image Processing) so that a concentration of a toner image to be newly generated on a surface of a photoreceptor drum and the like becomes high. The RIP means processing to generate print image data (bitmap) from PDL (Page Description Language) of 1 page using concentration correction information.
Such concentration correction information includes, for example, information for changing a dither pattern generated by RIP into a dither pattern whose painted-out area is larger.
When update of the concentration correction information is finished, the printing apparatus executes the RIP to generate print image data using the concentration correction information after update and executes printing processing of the print image data.
A printing execution data outputting system that executes such calibration is disclosed in Japanese Laid-Open Patent Publication No. 2006-082482, and an image processing system for executing such calibration is disclosed in Japanese Laid-Open Patent Publication No. 2006-287708.
Meanwhile, there has been a problem that at the time of executing aggregating printing (hereinafter, described as N-up printing), when calibration execution conditions are satisfied and a printing apparatus executes calibration, a difference is caused between an image concentration (also referred to as color tone) of a printed material of print image data generated using concentration correction information before update and an image concentration of a printed material of print image data generated using concentration correction information after update, which makes a user feel uncomfortable. Note that, N-up printing means processing in which print image data corresponding to a plurality of documents is aggregated into print image data corresponding to 1 page and the aggregated print image data is printed.
FIG. 14A and FIG. 14B are diagrams for specifically explaining such a problem. FIG. 14A is a diagram showing print image data corresponding to documents of 8 pages. FIG. 14B is a diagram showing print image data for 4-up printing (in the order of upper left, upper right, lower left, lower right) generated at the time of performing 4-up printing of the print image data shown in FIG. 14A.
When executing 4-up printing, a printing apparatus normally generates print image data sequentially from a first page so as to generate print image data corresponding to a document of 1 to 4 pages, print image data of 4 pages is aggregated to generate one print image data for 4-up printing as shown by a reference number P14 is generated.
Here, for example, assuming that at the time of finishing generation processing of print image data corresponding to a document in a second page as shown in FIG. 14A, calibration execution conditions are satisfied and concentration correction information is updated. At the time, a conventional printing apparatus uses concentration correction information after update in generating print image data corresponding to a document in a third page. The printing apparatus then generates print image data after the third page using the concentration correction information after update. Therefore, as shown by a reference number P15, in print image data on a same page, images whose color tones are different from each other are generated. When a printing apparatus prints the print image data, images whose color tones are different from each other are printed in a printed material of the same page, which may cause a user who looks this to feel uncomfortable.